The rapid identification of Pneumocystis carinii in respiratory specimens is critical for the optimal care of a variety of immunosuppressed patients, including in particular those with AIDS, who are especially subject to infection with this organism. A variety of staining techniques are already available, including toluidine blue and monoclonal antibody stains. The currently available stains are all moderately time-consuming to perform. A particular calcofluor white stain (Fungi Fluor, manufactured by Polysciences, Inc, Warrington, PA) has been found to stain P. carinii cysts. This stain is very simple to perform and might prove useful-particularly for centers which see relatively few P. carinii - infected patients. This staining procedure is currently being evaluated in parallel with more traditional methods to determine its sensitivity and specificity.